This invention relates generally to electromagnetic interference (EMI) feedthrough terminal assemblies, and related methods of construction, designed to decouple and shield undesirable electromagnetic interference signals from associated active implantable medical devices (AIMDs), such as cardiac pacemakers, implantable defibrillators, cochlear implants, neurostimulators, active drug pumps, and the like. More particularly, the present invention relates to an improved EMI feedthrough terminal assembly that includes an insulating shield to prevent damage or degradation to a feedthrough capacitor or its adjunct conformal coating from lead assembly methods, including laser welding, thermal or ultrasonic bonding or the like.
Feedthrough terminal assemblies are generally well known in the art for conducting electrical signals through the housing or case of an electronic instrument. For example, in AIMDs, the feedthrough terminal assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. In a cardiac pacemaker, for example, the feedthrough terminal pins are typically connected to one or more lead wires within the housing to conduct pacing pulses to cardiac tissue and/or detect or sense cardiac rhythms.
However, the lead wires can also undesirably act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the AIMD. It has been documented that pacemaker inhibition, asynchronous pacing and missed-beats can occur. All of these situations can be dangerous or even life threatening for a pacemaker dependent patient. In prior art devices, such as those shown in U.S. Pat. Nos. 5,333,095 and 4,424,551 (the contents of which are incorporated herein), the feedthrough terminal assembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple EMI signals to the grounded housing of the AIMD.
In general, the ceramic feedthrough capacitor has one or more passages or feedthrough holes and is connected to the hermetic terminal of the AIMD in a variety of ways. In order for the EMI filtered feedthrough capacitor to operate properly, a low-impedance, low-resistance electrical connection must be made between ground electrodes in the feedthrough capacitor and the ferrule of the feedthrough terminal assembly which in turn mechanically and electrically connects to the overall conductive housing of the AIMD.
For example, in a cardiac pacemaker, the feedthrough terminal assembly consists of a conductive ferrule generally made of titanium which is laser welded to the overall titanium housing of the AIMD. This not only provides a hermetic seal, but also makes the ferrule of the feedthrough terminal assembly a continuous part of the overall electromagnetic shield that protects the electronics of the AIMD from EMI. The ceramic feedthrough capacitor is, in turn, electrically and mechanically bonded to the ferrule of the hermetic terminal. In the past, and, in particular, as described in U.S. Pat. Nos. 5,333,095 and 4,424,551, the connection is typically performed using a thermal setting conductive adhesive. One such material is a silver flake loaded conductive polyimide.
The feedthrough terminal assembly consisting of the hermetic terminal with a mounted feedthrough capacitor contains one or more lead wires which must be connected to the internal circuitry of the AIMD. These circuit connections are typically done prior to the laser welding of the ferrule to the housing of the AIMD. Lead wire connections are performed in a number of ways, including flex cable connections, routing the lead wires to wire bond pads on a circuit board or substrate, or direct connection to hybrid chips of equivalent circuitry.
It is very important that surface insulation on the feedthrough capacitor and related terminal be in the hundreds or thousands of megaohms and be very reliable, rugged and stable. Degradation of insulation resistance or electrical failure of the feedthrough capacitor and/or the related feedthrough terminal assembly would lead to premature failure of the AIMD. For example, in the case of a cardiac pacemaker, the device could short out and cease its function in providing life saving pulses to the patient's heart.
In a higher voltage device, such as an implantable cardioverter defibrillator (ICD), it is extremely important that the surfaces of the feedthrough capacitor and related structures not be susceptive to high voltage electric arcing or flashover. When a ceramic feedthrough capacitor, which has a dielectric constant (K) of over 2000, operates at high voltage, there is a situation that is known as charge pooling which can occur on its surface. Where the high K ceramic dielectric material makes a transition to air, the dielectric constant changes suddenly from over 2000 to the permitivity of air which has a K of 1. This makes it very difficult for the electric fields to relax without creating microcolumb discharges, streamers or high voltage arcs. Accordingly, a common technique is to put a conformal coating over the top of the ceramic feedthrough capacitor. This can be in the form of a non-conductive polymer, such as a thermal setting epoxy or polyimide material. A convenient material for such purposes is a polyimide washer such as a thermal plastic polyimide supportive tape adhesive. One such material is manufactured by Ablestik (known as Ableloc® 5500). This material has a dielectric constant that is between 3 and 4 and serves to relax the high voltage fields.
However, it is very important that the conformal coating material not be damaged during installation of the feedthrough terminal assembly and its related components. Making lead attachments to circuits inside a pacemaker can cause heat, splatter or debris from the related coupling operations, i.e., welding, thermal or ultrasonic bonding, soldering, brazing, etc., to land on the capacitor's conformal coating. This heat, splatter or debris can cause localized melting or disruption of the conformal coating or embedding of metallic particles, all of which can lead to high voltage breakdown of the device. This can lead to immediate or delayed failure of the AIMD, which, as mentioned above, could be life threatening to a patient.
The conformal coatings that are used over feedthrough capacitors to prevent high voltage breakdown are typically quite thin. One reason for this is the need to keep AIMDs very small in size. Another reason is that these coatings do not have the same coefficient of thermal expansion as a barium titinate capacitor. Too thick of a conformal coating could lead to cracking of the dielectric in the feedthrough capacitor which can also cause reliability problems. Accordingly, there is a need for a design which will protect the surface of the feedthrough capacitor and/or its conformal coating from damage caused by lead attachment processes, including laser welding, thermal or ultrasonic bonding, soldering, brazing or the like.